Asked by: Stefania Papadopoulou, by email
All humans are built to the same basic female plan with the foetus developing as female or male depending on whether it has XX or XY sex chromosomes and hence different hormones. Testosterone causes a clitoris to grow into a penis and labia to become a scrotum. The mystery is that in many mammals male nipples are tiny, while in men they are fully formed with blood vessels and nerves. Quite why this should be no-one seems to know.